Thin film transistors that have been used for driving display devices have generally comprised amorphous silicon, polycrystalline silicon, or the like. Since these materials exhibit photosensitivity to light in the visible spectrum, carriers are generated by incident light, and resistivity of a thin film in such transistors is lowered. For this reason, when the transistors are irradiated with light, the transistors may switch to an ON state, despite a need for the transistor to be controlled in an OFF state. Accordingly, heretofore, to keep the transistors at the OFF state when needed, the lowering of the carrier resistivity of the thin films due to the radiation by light has been prevented by the use of a light shielding layer made of a metal film or the like.
Liquid crystal display devices in particular have been widely used for portable electronic devices such as notebook personal computers. Requirements for such displays include high luminance, miniaturization, and energy saving. To meet these requirements, it is necessary to increase the area ratio of an effective emitting portion to the total area of each pixel in a display. However, the presence of a light shielding layer in the transistor for driving the liquid crystal display device as described above reduces the area ratio (“opening ratio”) of the light transmission portion to the area of the light shielding layer in each pixel. Accordingly, a reduction of transistor area by improving performance of the transistor or an improvement of luminance of a backlight are necessary to develop a display device having high luminance. However, the measure to improve the performance characteristics of the transistor limits manufacturing yield, leading to an increase in cost. Moreover, the measure to improve the luminance of the backlight increases energy consumption. Thus, elimination of the need for a light shielding layer is desirable.
Double-injection field-effect transistors have been known for some years and their advantages over some other types of transistors are known. Heretofore, double-injection field-effect transistors have conventionally been made mostly from materials that are optically opaque.
In the field of displays and in other technological fields, there is a need for transparent transistors. Among the applications of displays employing transparent transistors are displays known as “heads-up” displays and “augmented reality” displays which allow a user to view a real environment beyond the screen of the display.
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